The Effect of Conversion Coated and Plated Components on the Corrosion of Cobalt Alloy Magnetic Disks

Common disk drive components with chromate conversion coatings or electroless nickel platings were analyzed by ion chromatography for residual levels of Cl^-, F^-, SO₄^2-, and NO₃^-. Extraction with boiling DI water revealed levels up to several hundred ng/cm² for these anions. When these components were placed 2 mm under a suspended cobalt-nickel-phosphorous disk in an environmental chamber at 55°C/85% RH, corrosion of the cobalt disk occurred directly over the chromated component when the total level of anions was greater than 200 ng/cm². This accelerated corrosion did not occur in a 120-h test when the level was less than 50 ng/cm². The amount of corrosion was found to be dependent on the separation distance between the disk and the chromated component.

Surface analysis of the disk corrosion area exposed to a chromate conversion coating showed NiO, CoO, and an increased concentration of chlorine.